After looking at the log files of my Magento site, I noticed that a URL ending .co.uk/no-route was getting a significant number of events.
What is this URL?
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Any URL in Magento which does not find target page those URL will be considered as a (no-route) 404 Page
In Magento, such pages are called no-route (means for which routing is not available)
Simply all 404 request will be considered as no-route URL.
Magento default installation already has a predefined custom
404 page (no-route).
Under Magento administration we can manually set default 404 page template.
System -> Configuration -> General -> Web -> Default Pages -> CMS No Route Page.
Here you can add any previously created CMS page (Customer Services for example).
404 page settings
Or you can use more specific content (search form, store sitemap, contact form, specific store categories with promoted products…).
I hope it will clear your concern.